The invention relates to a directionless and colorless reconfigurable optical add/drop multiplexer and to a method for dropping and adding an optical signal with a selectable wavelength at different clients.
In optical networks it is well known to use reconfigurable optical add/drop multiplexers (ROADMs). A reconfigurable optical add/drop multiplexer (ROADM) is a device that can add, drop, block, pass, equalize or redirect light waves of various wavelengths in a fiber optical network. Such ROADMs can be used in optical systems that transport, add, drop and/or redirect wavelength division multiplexed (WDM) signals. A ROADM allows wavelength selective switching without an optical to electrical or electrical to optical conversion. Outgoing wavelength can be generated in an add operation and an incoming wavelength can be terminated in a drop operation. A light wave can also be passed through the reconfigurable optical add/drop multiplexer without modification in a cut through operation. The ROADM is an optical add/drop multiplexer which provides the ability to remotely switch data traffic of a WDM system at the wavelength layer.
The reconfigurable optical add/drop multiplexer ROADM can be implemented in a conventional way by so-called wavelength selective switching WSS or by a planar lightwave circuit PLC devices. A M×1 WSS uses free-space optics using lenses, mirrors, Grating in conjunction with an optical beam processing device such as MEMS, Liquid Crystal, Digital Light Processing mirror pixels, and Liquid Crystal on Silicon pixels. A M×S WSS can be formed using one or more sets of the optical beam processing devices listed above. An M×1 WSS is used to form a line ROADM in optical network applications. When wavelength add/drop is necessary for a particular channel a light beam at the original wavelength is filtered out and its data extracted. Then, the data is impressed onto a light beam of same or another wavelength. Traffic can be re-directed to any direction when several line ROADMs are used. However, the add/drop functionality is localized (add/drop on the same direction only). However, there is a need for a centralized add/drop to any and every directions. In OFC 2008 Peter Roorda, “Evolution to Colorless and Directionless ROADM Architectures” a directionless and colorless ROADM has been proposed. However, this directionless and colorless ROADM has the disadvantage that once a wavelength is added or dropped from one direction it is blocked from being added or dropped to other directions. Accordingly, there is a need to overcome these limitations and to provide a ROADM which allows true optical branching for an optical network.